Technical Field
The present invention relates to flow through heater devices as used for example in coffee machines or other devices for preparing hot beverages and the like. In particular, the invention relates to a dynamic flow through heater having a helical shape, and a method of manufacturing such a helical flow though heater.
Description of the Related Art
In hot beverage preparation apparatuses like coffee machines, for example, electrical heating devices having an electrical power of several hundred watts up to more than 1 or 2 kW are used to produce hot water or steam for preparing a beverage. Controlling of such high heating power in order to produce a certain amount or a certain flow of water or steam having a specific temperature necessary to satisfy predetermined quality requirements is a challenging problem. One type of a common heating device comprises a kind of electrical heated massive thermo block having a relative heavy body made of aluminum, for example, through which the fluid is guided and which provides a high thermal inertia so that controlling the temperature of the fluid is relative easy because of the slow thermal reaction of the heating device in spite of the high electrical power. This device provides a uniform heat flow distribution and a relative constant fluid temperature when a slow switching controller is used for controlling the electrical heating power. However, the high thermal inertia results in a long heating-up period when activating the beverage preparation apparatus so that usability of the apparatus is affected. If, on the other hand, the heating device is operated in a stand-by mode so as to hold a predetermined higher temperature and shorten the heating-up period, then energy is wasted. Furthermore, a lot of heat energy is provided to the thermal block during the heating-up period and lost useless when the apparatus is switched of and cools down. Moreover, it is necessary to design a whole new special thermo block heating device for each application according to the requirements with respect to heating power, specific thermal load of surfaces and the like.
From DE 3542507 an electrical flow though heater is known, having a relative small thermal inertia and comprising an extruded aluminum profile having two adjacent channels. One channel receives a usual tubular heating element, while the other channel receives a stainless steel tube for carrying the water to be heated. The assembly of the extruded profile, the tubular heating element and the water tube is bent in direction of the smaller dimension of the cross-section of the extruded profile and wound to a helical shape so that the heating tube and the water tube are positioned on the same cylindrical surface. Thus, a relative compact heating device having a low thermal inertia is provided so that waste of energy and a heating-up period are reduced. That kind of heating device is called a dynamic flow through heater because of its fast thermal response that enables a short heating-up period and a dynamic temperature control.
However, manufacturing the heating device shown in DE 3542507 is laborious and expensive because first a complete tubular heating element has to be produced, and then the heating element and the water tube have to be slipped into the channels of the extruded profile before bending the assembly to the helical shape. Furthermore, the heat transfer from the heating coil of the tubular heating element to the water tube is impeded because the heat must traverse a high number of walls and interfaces, i.e., the interfaces arranged on the way from the heat-conductive insulating material (usually magnesium oxide) inside the tubular heating element to the wall of the heating element, the body of the extruded profile, and the wall of the water tube. Additionally, the surface contact between the tubular heating element and the extruded profile and between the extruded profile and the water tube is affected by the deformation of the profile and the tubes during bending to the helical shape. Hence, the surface contact and the heat transfer between these components may be reduced. Moreover, while heating up or cooling down, a different thermal expansion of the tubular heating element and the water tube leads to a torsional mechanical load and deformation during operation which can result in cracks created in the extruded profile or the tubular heating element. In order to avoid shortening of the operating life caused by that mechanical load, wall thicknesses have to be increased which is in turn undesired because thermal inertia and costs will be increased, too.
It is an object of the present invention to overcome the deficiencies of the prior art and to provide a dynamic flow through heater device having a very low thermal inertia and small dimensions while providing high heating power and an optimized heat transfer to the fluid to be heated. Another object is to provide a dynamic flow through heater device which is inexpensive and easier to produce. Another object is to provide a dynamic flow through heater device that can be easily resized in design in order to adapt it to varying application requirements. Moreover, it is an object of the present invention to provide a method of producing such an optimized dynamic flow through heater device in a reliable and cost efficient way.